Phase comparator



June 28, 1960` R. M. HATCH, JR

PHASE coMPARA'roR Filed Nov. s, 1957 A n j i PHASE `com?alzaron mhmdM-.lllcwll Jr., West Concord, assignor to the United States of America as represented by the Secretary of the AirForce Filed-Nov. 8,1957, ser. No. 69s,'4i1

" 1 claim. (Crausay l .l This invention Vrelates to aphase,comparator forv radio frequency signals.` Y n One obiect of the invention is to provide a phase comparator-which avoidsr180 ambiguity. Y

Another object is toprovide a phase comparatorwhich presents the data` instantaneously.

Another object is'to provide a phase comparator whose `accI'Jracyj-,ris independent ofthe phase.

Still another object is to provide a phase comparator whose accuracy is insensitive to frequency change over a small range.

These and other objects will be best understood from horizontal deliection plate 31 and vertical defiection plate 32 of a cathoderay tube and are simultaneously'applied to a phase detector 33. When the two signals'applied to the phase detector have exactly a 90 phase relationship the output from the phase detector 33 is zero. Deviation from the 90 relationship Will cause the output from the 1 phase detector by means of reactance tube 34 t'o conthe following description when read in conjunction with A the accompanying drawings, in which:

Fig. 1 is a circuit diagram in block'form of the phase V` comparator of this invention; and

Fig. 2 shows the face of the cathode ray tube of Fig. 1

`with a possible pattern shown thereon.

The input signal is shown in the following as nominally mc.; the intermediate frequencies are shown as 450 kilocycles, and 10 kilocycles. These numbers are merely used for examples; any other numbers would do as well. In fact, the input signal frequency herein shown as 30 megacycles may itself be an intermediate frequency preceded by 430 megacycle amplifiers, a UHF or microwave frequency local oscillator, and mixers.

With reference to -the block .diagram of Fig. 1, a 30 megacycle signal is applied through terminal 10 to mixer 12 togther with a 30.45 megacycle signal from Vlocal oscillator 14 through a buffer amplifier '13. Y The difference frequency, 450 kilocycles, is amplified by an intermediate frequency amplier 17. Simultaneously, a 30 megacycle signal of identical frequency with the first 30 megacycle signal but of unknown phase relationship is applied through terminal 11 to mixer 16 together with a signal from common local oscillator 14 through buffer amplifier 15. The output from mixer 16 will be of identical frequency with the output from mixer 12 and will bear the same phase relationship as the orignal 30 megacycle signals. The output from mixer 16 is applied to an IF amplifier 18.

A sample of the signal output of IF amplifier 17 is passed through an additional buffer amplifier 19 to a frequency discriminator20 which controls the reactance tube 21. The reactance tube 21in turn adjusts the frequency of the local oscillator 14 so as to maintain the IF frequency close to450 kilocycles', the. center frequency of the indenticalIF amplifiers 17 and '18.

The output from IF amplifier 17 is applied to mixer 22 togetherwith a` signal of approximately 460 kilocycles v vfrom oscillator 24 through a buffer. amplifier 23. The

t `difference frequency of approximately 10 kilocycles out of mixer 22,is amplified by amplifier 27 and applied to a phase shifting networkY 28. The two outputs from this phase shifting network should have exactly 90? phase difference and are connected respectively to a horizontal detrace.

trol fthe frequencyof'the oscillator 24. f The 'operation of this circuit will `then be -to 'maintain the frequency ofthe signal out of mixer 22, nominally 10 kilocycles, at; exactly that frequency, which produces by means of phase-shifting network 28, a 90 phaserrelationship be'- tween the signals applied to the deflection plates of the -oscilloscope tube, thus generating a trace on the scope screen which is an accurate circle.

-The signal out of IF ampliferl, referred to as the unknown phase, is applied rto mixer 26 together'with asignal from oscillator 24 through buffer amplifier 25. The signal out of mixerI 26 thus has a frequency identical to that of the signal out of mixer 23 and will bear a phase relationship identical to tha t between the two original 30 megacycle signals. The signal outof mixer 26 isapplied to an amplifier 35, squaring circuit 36, a differentiator 37, a'negative clipper 38, and a pulse amplifier 39. The output from the pulse amplifier 39 is thus a sharp positive pulse at the instantv of the positive going zero crossing of the sine wave applied to the amplifier 35.

This Ypulse Vis applied to a radial deflection anode 40 in the cathode ray tube causing a radial pip 011V the circular The operation of the overall circuit is such as to make the accuracy of the phase measurement independent of the Vexact input frequency. In practice, deviation of the input frequency from 30 megacylces of :1:2 megacycles was corrected by the automatic frequency control circuit 19, 20, and 21, so that IF frequencies were very nearly 450 kilocycles. In order for the accuracy of the phase indicator to be independent of the actual phase differenceV of the input signals, the trace of the scope must remain a perfect circle. This is accomplished by the automatic phase control circuit 33 and 34. If the oscillator 24 were a fixed frequency the difference signals out of mixers 22 and 26 would change slightly with changing input frequencies since the automatic frequency control'circuit does not completely correct for this frequency change. The signal out of mixer 22 would then change in frequency so that the phase network would not produce an accurate 90 phase shift and thus an elliptical trace. would be produced on the scope. .This effect is reduced suiciently in this apparatus by the automatic phase control circuit so that accuracy of measurement is limited only by the resolution of the trace on the scope screen.

The operation of the phasercomparator as designed and planned above is thus such as to produce a pip on the circular trace of the scope, Fig. 2, whose angular position isa measure of the relative phase of the signals applied 4to terminals 10 and 11 with an accuracy independent of the amplitude of this phase difference and, within limits, independent of the frequency of the input signals.

Further modifications of the invention concepts disclosed 'herein may now become apparentto those skilled Y Y tube,` having a horizontal deflection means,` a vertical y deflection means and a radial deflection means the relative phase between a reference radio frequency signaland a second radio frequency signal of the same frequency as said reference signal but of unknown phase, comprising; means for converting each of said radio frequency signals to an intermediate frequency signal with i mixer circuit,- means for applying said referencev interr.

mediate frequency signal to said iirst mixed circuit, means for applying the intermediate frequency signal of un- .known phase to said second mixer circuit, means for applying the youtput ofsaid intermediate frequency local Yoscillator to said first and said second mixer circuits to produce to low frequency signals with the same relative phase as said radio frequency signals, means responsive tothe output of said rst mixer circuit for producingV two signals one '90 out of phase with respect to the other, means for connecting said one signal to said horizontal deilecting means, means for connecting the other signal to the vertical deflecting means tothereby produce a cirjcular trace on the face of said cathode ray tube, a phase detector, means for connecting said two signals with the 90 phase relation to said phase detector, means responsive to the output of said phase detector, means for controlling the frequency of said 11F local oscillator to 4 preserve the 90 phase relation of said signals applied to horizontal and vertical deflection means, a clipper circuit connected to the output of said second mixer circuit to convert the low frequency signal of unknown phase to a substantially square wave, means for diierentiating said square wave and means for applying only one of the pulses of the diiferentiated signal to said radial deecting means to thereby produce a pip on the circular trace to indicate Vthe relative phase of A'said two radio frequency signals.

References Cited in the file of this patent UNITED STATES, PATENTS 2,320,476 Schrader June 1, 1943 2,422,386 Anderson June 17, 1947 2,481,659 Guanella Sept. 13, 1949 2,576,499 Bowes Nov. 27, 1951 2,580,803 Logan Ian.-1, 1952 2,588,094 Eaton Mar. 4, 1952 1614221 M011 oct. 14, 1952 2,793,347 Clark May 21, 1957 FOREIGN PATENTS 892,611 Germany Oct. 8,3953 

